1. Field of the Invention
The present invention relates to a sheet conveying apparatus, an image forming apparatus, and an image reading apparatus, particularly to a configuration for correcting skew feeding and crosswise misalignment of a sheet, such as recording paper and an original, which is conveyed to an image forming portion or an image reading portion.
2. Description of Related Art
Conventionally, the image forming apparatus and image reading apparatus such as a copying machine, a printer, and a facsimile include a sheet conveying apparatus that conveys the sheet such as the recording paper and original to the image forming portion and image reading portion. Sometimes the sheet conveying apparatus includes a skew feeding correction portion that corrects the skew feeding of the sheet and a displacement of the sheet in a direction orthogonal to a sheet conveying direction (hereinafter, referred to as crosswise direction) in order that an attitude and a position of the sheet are fitted until the sheet is conveyed to the image forming portion or image reading portion.
Recently, various sheets such as coated paper, emboss paper, extra thick paper, and extra thin paper are used in the image forming apparatus and the image reading apparatus. Therefore, in the image forming apparatus and the image reading apparatus, in addition to high productivity, a demand for speed enhancement and high accuracy of a skew feeding correction portion arises in order to deal with various sheets used.
In order to achieve the speed enhancement and high accuracy of the skew feeding correction portion, for example, U.S. Pat. No. 6,663,103 discloses an active skew feeding correction type skew feeding correction portion that corrects the skew feeding while the sheet is conveyed without tentatively stopping the sheet.
FIG. 17 illustrates a configuration of a conventional skew feeding correction portion. Referring to FIG. 17, a pair of skew feeding correction rollers 25a corrects skew feeding of a sheet S, and a pair of registration rollers 25b corrects a crosswise displacement of the sheet S (hereinafter, referred to as crosswise registration). As illustrated in FIG. 18, the pair of skew feeding correction rollers 25a and the pair of registration rollers 25b include conveying rollers 25a1 and 25b1 and driven rollers 25a2 and 25b2, respectively. The conveying rollers 25a1 and 25b1 include outer circumferential surfaces, and the driven rollers 25a2 and 25b2 are brought into pressure contact with the conveying rollers 25a1 and 25b1.
In cases where the skew feeding of the sheet S is corrected in the skew feeding correction portion, when a sensor 26 detects the skew feeding of the sheet S, rotation speeds of driving motors M1 and M2 that drive the pair of skew feeding correction rollers 25a is controlled to drive the pair of skew feeding correction rollers 25a at a speed according to a skew feeding amount of the sheet S. Therefore, the skew feeding of the sheet S is corrected.
Then the sheet S in which the skew feeding is corrected is conveyed to the pair of registration rollers 25b. The pair of registration rollers 25b is coupled to a coupling shaft 31 that is drive by a driving motor M3. A crosswise registration sensor 30 is disposed on a downstream side of the pair of registration rollers 25b in order to detect the crosswise registration. When the crosswise registration sensor 30 detects the crosswise registration, the driving motor M4 moves the coupling shaft 31 in the crosswise direction according to a crosswise registration amount of the sheet S. Therefore, the pair of registration rollers 25b is moved in the crosswise direction, and the crosswise registration of the sheet S is corrected.
When the pair of registration rollers 25b corrects the crosswise registration, the pair of skew feeding correction rollers 25a is avoided from becoming a resistance. Specifically, in correcting the crosswise registration, as illustrated in FIG. 18B, a conveying roller 25a1 of the pair of skew feeding correction rollers 25a that is rotated in a direction of an arrow is controlled such that an outer circumferential surface 33 is located at a cut-out position.
After the crosswise registration is corrected, the sheet S is conveyed to the conveying belt 10, and the conveying belt 10 conveys the sheet S onto the downstream side. The conveying belt 10 avoids the pair of registration rollers 25b from becoming the resistance against the sheet S. Specifically, in conveying the sheet S, as illustrated in FIG. 18B, a conveying roller 25b1 of the pair of registration rollers 25b is controlled such that the outer circumferential surface 33 is located at the cut-out position.
When the conveying rollers 25a1 and 25b1 are located at the cut-out position, the driven rollers 25a2 and 25b2 that are brought into pressure contact with the conveying rollers 25a1 and 25b1 drop in a pressurizing direction, and the driven rollers 25a2 and 25b2 are projected into a conveying guide path. When the driven rollers 25a2 and 25b2 are projected into the conveying guide path, the driven rollers 25a2 and 25b2 become a conveying resistance against the sheet S.
Conventionally, a mechanical link mechanism (not illustrated) moves the driven rollers 25a2 and 25b2 in a direction in which the driven rollers 25a2 and 25b2 are separated from the conveying rollers 25a1 and 25b1 at the positions where the outer circumferential surfaces 33 of the conveying rollers 25a1 and 25b1 are cut out. That is, the mechanical link mechanism separates the driven rollers 25a2 and 25b2 in synchronization with phases of the conveying rollers 25a1 and 25b1. This enables the sheet skew feeding and the crosswise registration to be continuously corrected.
However, in the conventional sheet conveying apparatus, image forming apparatus, and image reading apparatus, after the driven roller is separated in synchronization with the phase of the conveying roller, the driven roller is brought into pressure contact with the conveying roller in predetermined timing by a biasing force of biasing means (not illustrated). When the driven roller is brought into pressure contact with the conveying roller, a shock is applied to the driven roller, and vibration is generated in the driven roller as illustrated in FIG. 19.
The vibrations are not always generated at the same time in the driven rollers provided in the crosswise direction. Sometimes the driven rollers 25b2 of the pair of registration rollers 25b are vibrated in a back and forth direction with respect to the sheet conveying direction as illustrated in FIG. 20A, and sometimes the driven rollers 25b2 are vibrated in the opposite direction to the pressurizing direction as illustrated in FIG. 20B.
When the sheet is conveyed to a nip of the pair of registration rollers 25b before the vibration of the driven roller is attenuated, a displacement is generated in the sheet conveying direction by the pair of registration rollers 25b, and a nip pressure of the pair of registration rollers 25b becomes unstable. In such cases, random skew feeding is generated each time the sheet in which the skew feeding is corrected is nipped between the pair of registration rollers 25b. Particularly, in the case of thin sheet such as 37-g to 52-g paper, the sheet is easily influenced by the nip pressure of the pair of registration rollers 25b, the random skew feeding (variation in skew feeding) is remarkably generated.
In order to reduce the shock in bringing the driven roller into pressure contact with the conveying roller, it is necessary that component accuracy of the mechanical link mechanism be improved to finely adjust a separation amount and attachment/detachment timing of the driven roller as much as possible. However, the mechanical link mechanism becomes complicated and a huge amount of time is required for the adjustment, which results in large cost increase.
Because an elastic roller (rubber roller) is used on at least one of the driving side and the driven side of the pair of registration rollers 25b, duration abrasion is generated, and the separation amount is easily changed according to the duration abrasion. Therefore, it is necessary to frequently exchange the rollers, and a work load on a service person is increased because the adjusting work is generated in each roller exchange in the field.
The present invention has been made in view of these circumstances, and an object thereof is to provide a sheet conveying apparatus, an image forming apparatus, and an image reading apparatus, in which the generation of the skew feeding caused by the shock in the pressure contact of the pair of rotation bodies can be reduced.